With such sealing tools, for example in deep-drawing packaging machines or tray sealing machines (“tray sealers”), packages are hermetically closed and sealed. To extend the storage life of the packaged products, the package can be optionally closed under a modified atmosphere, where the air in the package is evacuated or at least partially replaced by an exchange gas.
In most cases, a sealing plate is provided as a sealing tool inner part which is in connection with a heating plate or another heating means to be heated to the desired sealing temperature. The sealing plate, i.e. the sealing tool inner part, is movable in the vertical direction to be lowered onto the package and pressed against the package. If compressed air at a sufficiently high pressure is available, for example with at least 6 bar, the sealing plate is driven by means of pneumatic cylinders or inflatable hoses which are located on the upper side of the sealing plate and are filled with compressed air.
However, if no compressed air supply or a compressed air supply at a maximum overpressure of only 1 bar with respect to normal pressure is available, for example from a vacuum pump in compressor operation, no sufficient sealing pressure can be achieved any more with such a drive. Even if the sealing chamber is evacuated, the maximum pressure difference in this case is only 2 bar.
For such cases of a compressed air supply with insufficient pressure, according to the formula “force=pressure X area”, it makes sense to increase the area subjected to pressure to achieve sufficiently high sealing forces. To this end, either the complete surface of the sealing plate can be subjected to pressure, or the sealing plate is rigidly connected to a piston plate disposed outside the actual sealing chamber. In this case, the pressure acts on the piston plate, and the piston plate can be considered as the movable sealing tool inner part whose movement is transmitted to the actual sealing plate.
If the upper side of the complete sealing plate or a piston plate is subjected to pressure and is to be moved thereby, sufficient sealing is required during the complete movement. For this purpose, it is known to insert a flexible O-ring seal into the gap between the sealing tool outer part and the movable sealing tool inner part. As an alternative to this, a membrane can be provided (except for optionally small recesses for passing through lines), whose surface is larger than the surface of the sealing plate or of the piston plate and which is clamped outside the sealing tool inner part all around the sealing tool outer part. If this large-surface membrane is subjected to pressure from above, it will lie against the sealing plate or the piston plate, respectively, and presses the latter downwards.
However, such a membrane has a disadvantage in that it is highly stressed and has to be replaced frequently, resulting in a standstill of the sealing tool, and thus in the worst case also of the complete packaging machine.